The extent of alpine glaciation in the Wasatch Mountains, and location of Little Cottonwood Canyon (yellow star). The contours are in feet. This map is in the general collection of Don Currey. I tracked down the authorship to a 1911 source.

A classic U-shaped valley (dashed line) is legacy of the Pleistocene glaciers emanating from Little Cottonwood Canyon. Can you denote the trim line along the northern (left) side of the valley? The multi-million dollar homes depicted happen to straddle the Wasatch fault, which remains active today. Kathleen Nicoll 2007.

An oblique image looking south towards Lone Mountain in the Wasatch Range. You can notice the sinuous riverbed for the river depicted in Figure 4, and several hanging valleys (outlined with dashed lines). Google Earth and Utah Geological Survey (Inset).

Figure 10 Tabular rocks are part of a large rock slide//avalanche that seems to have originated from the south-facing slope. Kathleen Nicoll 2007.

Description

The Central Wasatch Mountain Range provides a dramatic and scenic backdrop to the Salt Lake City urban corridor, with peaks rising to heights of over 3350 m (~11,000 ft). The Wasatch is an eastward-tilted fault block range, bound on the west by a normal fault system that has been active since ~30-17 million years ago. As a result of uplift, deep canyons have been carved into the west side of the Wasatch range. Some of these canyons were glaciated during the last Ice Age (Figure 1). During the Last Glacial Maximum until ~18,000 years ago, ice from Little Cottonwood glacier may have emptied directly into the ancient Lake Bonneville (Madsen and Currey, 1979). This virtual tour up Little Cottonwood Canyon (Figure 2) allows the geo-savvy observer to notice some classic geomorphic attributes and the sediments associated with Quaternary fluvial, glacial and colluvial deposits.

At the mouth of Little Cottonwood Canyon, facing east, the canyon's U-shape attests to its glacial origin (Figure 3). During the Pleistocene, this area was the exit point of a large west-flowing trunk glacier into the Salt Lake Valley. Along the south side of the valley, the classic U-shape is slightly incised by a V-shape due to recent fluvial incision. Figure 4 shows the Little Cottonwood Creek at a dry stage during the summer, when you can easily see the sediments that typically make up its bedload. You will want to compare (and contrast) the size and shape of these sediments with those we will virtually visit locales in the canyon.

Travelling up the canyon via highway UT- 210, we parallel the Little Cottonwood Creek for 5 miles. Classic glacial features are apparent along the south side of the valley. Little Cottonwood Valley preserves evidence of at least 10 glacial cirques and hanging valleys, several of which are compound. The oblique aerial image in Figure 5 is annotated to highlight the location of some hanging valleys, which were left behind as the ice melted; these were probably exposed around 13,000 years ago.

At the top of the valley, above the Alta Ski Resort, we notice several moraine lobes that span the valley (Figure 6) downslope of the Albion Basin. These rocks are characteristically large cobbles and boulders that may be quite blocky or angular. Glacial erratics scattered throughout the Little Cottonwood Valley are commonly massive, and may be 5-10 meters (Figure 7).

Driving back down the valley to the location of Tanner's Flat (Figure 8), we notice that the northern side of the canyon (which is the south-facing side of the canyon) is quite steep. The north-facing aspect of the canyon is much more dissected, and it was formerly glaciated; the ice left behind hanging valleys, cirques, and arêtes. At Tanner's Flat, we stop to examine local features; figure 9 depicts the valley floor, looking down from the north side of the Canyon. What is the area outlined by the dashed line?

Figure 10 shows the sediment that makes up the feature outlined in Figure 9. This rock pile is a beloved bouldering location. How did it form?

A likely interpretation is that the rock pile at Tanner's Flat resulted from a prehistoric avalanche. The evidence supporting this inference includes: 1) the regular tabular form of the individual rocks making up the pile suggests fracturing and a colluvial transport mechanism for the talus; 2) the lack of rounding of the blocks suggests that the rocks have not been modified by running water as would be typical in a mid-valley fluvial environment; and 3) the position of the pile is upstream of the hanging valley depicted in Figure 9, eliminating the possibility of glacial ice transport for the rocks.

These reasons outlined above suggest that an avalanche affecting Tanner's Flat might have originated along the south-facing slope. On Figure 9, can you find a candidate "chute" that might have been an active sediment corridor for the rock slabs in the pile at Tanner's Flat? What might have caused a rock fall or an avalanche at this locality? Can you speculate as to how fast this deposit might have formed?